1. Field of the Invention
This invention relates to an alarm circuit which detects combustion products such as smoke, vapor and the like by the use of an ionization smoke sensor having a high impedance. More particularly, it relates to a large number of high impedance alarm circuits connected to a common d.c. power source independently of one another, in which power to be dissipated on the basis of impedance changes smaller than an impedance change at a predetermined alarm generating level of the sensor is reduced and in which a required operating voltage is stably supplied.
2. Description of the Prior Art
U.S. Pat. No. 3,733,596 discloses an alarm system in which a normalized normal voltage is applied to ionization smoke sensors having a high impedance and in which a free running multivibrator (FMV) is employed in order that a certain impedance change may be normally bestowed on a field effect transistor (FET) by a measurement output of the sensor (refer to FIG. 1). The FMV is a pulse switching unit, which produces a continuous signal train of intermittent pulses. The duration of the output pulses provided at relatively long quiescent intervals is made shorter. Within the period of time during which the output pulse is impressed, the impedance change of the ionization smoke sensor ascribable to combustion products such as smoke having entered thereinto is transmitted to the FET. Although the use of the intermittent pulse train can reduce the power dissipation, it is empirically known that a combustion product detecting circuit which exploits an ionic current maintaining a high impedance can have the reliability of the detection lost. The FMV adopted in the prior art employs FET's at the input and output ends thereof, and the intermittent pulses to be produced thereby cannot satisfactorily be stabilized under the present technological situation. Due to the synergy between such comparatively unstable pulse train and the influence of a fluctuation of the intensity of the applied voltage on the ionic current, the loss of the detection accuracy (for example, a case where the detecting operation was not effected in spite of the presence of the combustion product, or a case where it was effected in spite of the absence of the combustion product at the alarm level) may possibly have been experienced.
The FET having received the impedance change of the ionization smoke sensor delivers an amplified current corresponding thereto to a transistor, and the signal further amplified by the transistor triggers a thyristor. The thyristor short-circuits a power source, and a relay incorporated in a short-circuiting loop is energized for the first time by a current increased by the short-circuit. The relay actuates an alarm sunding or displaying circuit which is separately constructed.
FIG. 2 shows a comparator circuit wherein the foregoing FMV is removed which is not always apprehended to be favorably combined with the high impedance circuit involving the ionic current changing under the external influences. In this example for reference, to the end of reducing the power dissipation, a zener diode and a resistance are connected in series to the drain of the FET subject to the impedance change of the ionization smoke sensor, and the operating condition of the zener diode is determined in correspondence with the alarm issuing level. A transistor adapted to be rendered conductive by a voltage which appears across the resistance when a zener current develops is connected to the juncture between the zener diode and the resistance. A thyristor which is triggered by the "on" operation of this transistor constitutes the shortcircuiting loop described above. However, the reduction of the power dissipation owing to the zener diode is subject to an apparent limit, and the number by which the high impedance circuits can be connected to the common d.c. power source independently of one another is restricted. The thyristor of the shorting circuit is not rendered conductive by impedance changes which do not come up to the alarm issuing level. Since, however, the slight zener currents corresponding to the impedance changes below the predetermined level exist, the current leakage of the transistor Q is of an unnegligible amount. The leakage current of the transistor increases suddenly as the impedance change approaches to the alarm issuing level. Even in the absence of any cause for a fire, the impedance of the ionization smoke sensor continues to sensitively vary due to other factors. Actually, therefore, the impedance changes close to the alarm generating level determined for avoiding false alarms arise more frequently than anticipated.
As the number of the high impedance circuits having the ionization smoke sensors connected to the common d.c. power source is larger, the increase of the leakage current occurring in the transistor as corresponds to the impedance change exerts a greater influence on the operating conditions necessary for the circuits, so that the voltage to be applied across an inner electrode 1 and an outer electrode 5 of the ionization smoke sensor lowers to the extent of losing the normal detecting function. When, as in a fire of smoldering fibrous materials in which the amount of smoke increases very slowly, a plurality of ionization smoke sensors are exposed to the gradually increasing smoke for a long period of time before the alarm generating level is reached (this is encountered in, for example, a warehouse), the voltage to be impressed across the inner electrode 1 and the outer electrode 5 lowers considerably to spoil the detecting function of the sensors because it is divided by the impedance of the transistor Q, a resistance R.sub.1 and the detecting resistance of a receiving unit (not shown), in addition to the cause of the leakage current increased in the transistor Q. Even when the quantity of the combustion product to detect the impedance change of the alarm generating level is thereafter reached in actuality, no detection output is provided from the circuit in some cases.